Under which condition is a river more likely to expand its floodplain?

Correct answer: Gentle slope, frequent floods and lateral erosion

The direct answer is B: a gentle slope, frequent floods, and lateral erosion make floodplain expansion more likely. A floodplain is the fairly flat land beside a river. When the slope becomes gentle, the river flows more slowly and cannot cut deeply downward as easily. Instead, it bends from side to side, erodes its banks, carries sediment during floods, and deposits some of that sediment on nearby land. Repeated flooding and sideways erosion gradually widen the flat valley floor. Option A is wrong because a steep slope usually makes water flow fast and encourages deep vertical cutting, not broad floodplain growth. Option B is correct because all three conditions support sideways erosion and deposition. Option C is wrong because merely freezing a glacier does not explain ordinary floodplain expansion. Option D is wrong because distance from the sea is not the controlling condition; floodplains can develop inland. Memory cue: gentle slope plus repeated floods means sideways spreading and a wider floodplain.